Castle: Pure Lines and Selection 



97 



nothing about the appearance of its 

 leaves, stem, or root; we know nothing 

 of its physiological behavior under con- 

 trolled environment; yet we are asked 

 to believe the two plants, of 50 years 

 ago and of today, identical in genetic 

 character, because a selected head of one 

 can be roughly matched with a selected 

 head of the other. Does this prove that 

 selection is without effect? Rather it 

 proves that wheat can be selected to a 

 type. What evidence have we that 

 any attempt has been made in these 

 50 years to change the type? Not once, 

 I venture, has the shortest headed plant 

 been chosen in the long headed variety, 

 or the least branched head in the 

 branched headed variety. Yet on the 

 pure line hypothesis, this would have 

 made no difference in the result. How 

 do we know that it would have made no 

 difference, the experiment not having 

 been made? Doubtless the pure line 

 advocate would reply, because selection 

 in the opposite direction has effected 

 no change, the long headed race being 

 now no longer and the branched headed 

 race no branchier. But this assertion 

 is not established by the photographs of 

 matched heads, and even granting its 

 correctness, it might well be that a 

 physiological limitation had been reached 

 50 years ago which it has not been since 

 possible to remove. When selection 

 has attained the ultimate, it can go no 

 farther. If I select animals spotted 

 with white for increasing blackness and 

 obtain an animal black all over, the 

 task is accomplished. Nothing more 

 can be done in that direction. This does 

 not prove selection to increase or to 

 decrease the white spots of animals to 



be ineffectual; indeed, the contrary has 

 been fully established. 



The Hagedoorns think further to 

 strengthen the argument for the ]jure 

 line theory by a challenge to its oppon- 

 ents to modify a character within a 

 narrowly inbred line of animals by se- 

 lection and then to bring the character 

 back again to its original condition. 

 "For," they say, "it is obvious that it 

 may be possible by selection to get rid 

 of such genetic factors as influence the 

 character selected in a direction oppo- 

 site to that in which we want to modify 

 it, and thus to make great progress. On 

 the contrary, if selection really might 

 change genetic factors, modification 

 of any character would be as easy in one 

 direction as in the other." This test 

 (in all essential features) has been ap- 

 plied repeatedly in a series of selection 

 experiments with the hooded pattern 

 of spotted rats (a Mendelizing unit- 

 character) described in Publication No. 

 195 of the Carnegie Institution of Wash- 

 ington.^ The experiments show that 

 plus selection and minus selection are 

 both effective, and that selection away 

 from and back to any modal condition 

 of the pattern are equally easy of accom- 

 plishment. 



If I were disposed to use challenges 

 for argument, I would make a counter 

 challenge in these words — Let him who 

 thinks selection ineffectual try it on any 

 character of any animal. If he is per- 

 sistent and critical in his experiments, 

 I am confident that he will be convinced 

 of the effectiveness of selection, as all 

 practical breeders are, and will thence- 

 forth regard "pure lines" as purely 

 imaginary. 



^A preliminary account of certain of the experiments was given in Am. Breeders' 

 Vol. 3, No. 4. 



Magazine, 



R. H. JOHNSON IS CHAIRMAN 



Professor Roswell H. Johnson of the University of Pittsburgh has been ap- 

 pointed chairman of the Eugenics Section of the American Association for the 

 Study and Prevention of Infant Mortality, which will hold its next meeting at 

 Boston in November. 



